Lead plating bath



Patented Apr. 5, 1949 UNITED STATE ATENT OFFICE LEAD PLATING BATH No Drawing. Application November 26, 1945, Serial No. 630,989

7 Claims. 1

plating, the Betts process substantially as origi- 3 nally used with the fluorine compound electrolyte is presentlygencrally applied commercially to the substantial exclusion of other methods.

I have made a startling discovery enabling the production of lead films favorably comparing with any produced by heretofore known electrolytic processes and not involving either fluorine-containing substances or the careful regulations and controls of temperature essential in present popular processes. My discovery furthermore enables the use of aqueous electrolytes consisting of abundant and inexpensive substances which require no special processing prior to use. Also, as a result of my discovery and in accordance with the process of my invention based on that discovery, a further advantage over the Betts process is the elimination of the necessity for establishing a preliminary copper coating upon a ferrous metal surface to which a lead film is to be applied. A still further advantage of my process over those of the art is the substantially non-corrosive nature and effect of the electrolyte of my present invention because of its high pH value.

Specifically, I have found that electrolysis of a suitable aqueous solution containing lead acetate, gelatin or bone glue or similar substances and a water soluble aromatic sulfonic acid will result in the formation of a smooth, fine-grained, uniform and tenaciously adhering lead film upon a suitable metal surface such as oxide-free ferrous metal.

Briefly, the rocess of the present invention comprises exposing a suitably prepared metal surface as a cathode to the aqueous composition of the present invention in contact with a lead anode and containing lead acetate, gelatin and a water-soluble aromatic sulfonic acid, while fr'naintai'ning a suitable electric current density in said solution. I

2 Herein and in the appended claims sulfonic acid is used generically in that itmeans-and includes free sulfonic acids and their salts which result upon replacement of the hydrogen atom of the sulfo or other acid group with an organic or inorganic salt-forming radical or element. The term polycyclic is used in the accustomed manner of the art and means and includes com-- pounds of the naphthalene and related series as distinguished from compounds of the benzene series.

In accordance with a preferred form "ofthis invention a lead piece and a ferrous'm'etalshape having a suitably prepared surface are immersed as anode and cathode, respective, in the'solution of this inventionwhich has a pH of about 5.0 is maintained at a temperature of 150 Rand is impressed with a current density of about amps per square foot of cathode surface until a lead film of the desired weight covers "the ferrous metal surface. During the period of immersion and electrolysis, the electrolyteissubjected to mild agitation in order to prevent localized substantial depletion'of lead'ions in the vicinity of the cathode element, that is, the surface being plated, and localized substantial overconcentration of lead ions in the vicinity of lead anode. Thus uniform corrision of the anode and anode and uniform cathode accretion is obtained.

The temperature of the electrolyted'uring this process may vary over a broad range without impairing the eifectiveness of the operation,"but in the interest of efficiency it should be maintained at'b'etween about 120 F; and 160 Ffduring electrolysis.

As in electrolytic processes in general," the current density in the electrolyte in the process of this invention may be varied over a wide 'range to yield satisfactory results. However, it is my preference in practical commercial operation, after comparing the efficiencies of operations in accordance with this invention over a wide range of current densities, that the current density in this process be between about 50 and about amperes per square foot of cathode surface. Also, it is my preference that the current density be maintained substantially constant during electrolysis. When the current density is in excess of about 100 amperes per squareffoot of cathode surface, gentle agitation as aforementioned and for the above mentioned reasons is desirable and in some instances essentialif a satisfactory lead coating is to be produced. Such agitation of the electrolyte is not essential but not undesirable when current density is less" than about 100 amperes per square foot of cathode surface.

For a more strongly adhering, uniform and otherwise desirable film of lead, it is occasionally necessary to prepare the surface upon which the lead film is to be established. Often such preparation is essential to the establishment of any coherent and adherent lead film as in the case of oxide-coated ferrous metal. In accordance with my preferred practice as applied to surfaces of shapes of ferrous metal, then surface is contacted with an aqueous solution of sulfuric acid having a temperature of about 200 F. for between about 30 and about 60 seconds, and then briefly contacted with an aqueous acetic acid solution at about 160 F. The acids may suitably be of about 8% to 12% strength in accordance with conventional pickling practice. In the sulfuric acid treatment sufficient time should be permitted for action upon and removal of the oxide scale and organic matter which will vary from surface to surface, but in any event more than 60 seconds should not be required for completion of the acid action. Uneconomical loss of metal and depletion of acid strength will result from overlong delay in removing the metal shape from the sulfuric acid solution.

The aqueous acetic acid solution is used as a rinse to accomplish removal of adhering sulfuric acid and materials not entirely removed by the sulfuric acid solution. Time is not usually a critical factor in this step inasmuch as corrosion of the ferrous metal surface by acetic acid of this strength is negligible. Time is, however, important when the articles to be lead plated are being transferred from the acetic acid solution to the electrolyte. Preferably that period is as abbreviated as possible in order that oxidation of the metal surface upon contact with air is minimized. When this transfer period is long enough for the aqueous acid film on the steel to dry, a deposit is formed which has the appearance of a red stain. The plating adheres poorly to the steel on areas covered by that stain.

The electrolyte is comprised, as aforementioned, of lead acetate, elatin, or bone glue or similar substances and water-soluble aromatic sulphonic acid and consequently has a relatively high pH. My preferred composition, however, contains in addition substantial quantities of sodium acetate, acetic acid and a lesser amount of sucrose. A particular electrolyte solution which I have successfully employed consisted of 220 grams per liter of lead acetate, 100 grams per liter of sodium acetate, 15 grams per liter of acetic acid, 4 grams per liter of bone glue, 5 grams per liter of sucrose and 5 grams per liter of 2-naphthol-6-sulionic acid. I have discovered, however, that these electrolytes may desirably contain varying amounts of these materials as follows:

Between about 200 grams and about 225 grams per liter of lead acetate; between about 60 and about 300 grams per liter of sodium acetate (anhydrous basis); between about 15 and about 60 grams per liter of glacial acetic acid or equivalent amount of weaker acetic acidi between about 4 and about 6 grams per liter of bone glue; between about 5 and about 10 grams per liter of sucrose; between about 4 and about 10 grams of water-soluble polynuclear aromatic sulfonic acid.

No particular correlation between the amounts of these various ingredients has been found necessary to the commercially satisfactory production of suitable films of lead. It is, however,

desirable that the amounts of these ingredients be within the above ranges for otherwise poorer results will be obtained upon electrolysis.

In these electrolyte compositions of my invention the various common gelatins, including bone glue, are suitable singly or in any combination with each other. Also, it is not essential that sucrose or equivalent be included, but in the event it is used care should be exercised to limit the amount below that which will cause undesirable foaming.

Satisfactory results are obtained with various water-soluble aromatic sulfonic acid substances including:

2-naphthol-6-sulphonic acid 2-naphtl1ol-7-sulphonic acid 2-naphthol-8-sulphonic acid Z-naphthol-S-sodium sulfonate 1,5-naphthalene, disulphonic acid 1,6-naphthalene disulphonic acid B-aminodjmaphthale'ne disulphonic acid 4-amino-1,5-naphthalene disulphonic acid -amino-l-naphthalene disulphonic acid 2-naphthol-3,6-disulphonic acid Z-naphthol6,8-disulphonic acid l-naphthol-Z-sulphonic acid 1-naphtho1-3-sulphonic acid l-naphthol--sulphonic acid l-naphthol-5-sulp-honic acid l-naphthol-G-sulphonic acid l-naphthol-l-sulphonic acid l-naphthol-S-sulphonic acid These substances are water soluble within the meaning of that term as herein used to specify an appreciable solubility in water at F., i, e. solubility to the extent of at least 5 grams per liter.

These compositions of the invention possess a relatively high pH as aforementioned, preferably above about 4.5 and as high as 6 in certain circumstances.

The following example, which is illustrative only and not limiting in any respect, embodies the present invention as I have actually carried it out and is offered in order that those skilled in the art may be thoroughly informed as to the substance of this invention.

A steel sheet was immersed for 30 seconds in a 10% aqueous solution of sulfuric acid having a temperature of 200 F., then immediately immersed in a 10% aqueous solution of acetic acid having a temperature of 150 F., and was then -:-.irectly transferred to and immersed in an aqueous electrolyte having a temperature of F.

and containing 220 grams per liter of lead acetate,

100 grams per liter of sodium acetate, 15 grams per liter of acetic acid, 4 grams per liter of bone glue, 5 grams per liter of sucrose, 5 grams per liter of 8-amino-1-naphthol-3,6 disulphonic acid. An electrolytic current density of amperes per square foot of cathode surface was impressed upon this electrolyte causing corrosion of a lead anode also immersed in the electrolyte in close proximity to said steel sheet surface and deposition of elemental lead upon a surface of said sheet. During electrolysis the electrolyte was caused to circulate by means of a mechanical stirring device to equalize the concentration of lead ions throughout the electrolyte. Upon this completion of the electrolysis, the sheet bore a uniform fine-grained film of lead which was 0.0005 thick and which was difficult to remove with a sharp instrument and did not break before the steel sheet in a tensile strength test upon the Olson machine.

Because of the high pH of the electrolyte (4.9) there was no corrosion of equipment. Cost of the electrolyte per gallon was approximately onehalf that of the conventional Betts process 8160- trolyte.

Ratios, percentages and proportions stated herein and in the appended claims refer to the weight rather than the volume basis unless the contrary is specifically stated.

Having thus described the present invention so that others skilled in the art may be able to understand and practice the same, I state that what I desire to secure by Letters Patent is defined in what is claimed.

What is claimed is:

1. An aqueous electrolyte for use in lead plating consisting essentially of between about 200 and about 225 grams per liter of lead acetate, between about 60 and about 300 grams per liter of sodium acetate, between about 15 and about 60 grams per liter of acetic acid, between about 4 and about 6 grams per liter of bone glue, and between about 4 and about grams per liter of a water-soluble naphthalene sulfonic acid.

2. An aqueous electrolyte for use in lead plating consisting essentially of between about 200 and about 225 grams per liter of lead acetate,

between about 60 and about 300 grams per liter of sodium acetate, between about and about 60 grams per liter of acetic acid, between about 4 and about 6 grams per liter of bone glue and between about 4 and about 10 grams per liter of 8-amino-l-naphthol-3,6-disulfonic acid.

3. An aqueous electrolyte for use in lead plating consisting essentially of between about 200 and about 225 grams per liter of lead acetate, between about 60 and about 300 grams per liter of sodium acetate, between about 15 and about 60 grams per liter of acetic acid, between about 4 and 6 grams per liter of bone glue and between about 4 and about 10 grams per liter of 2- naphthol-B-sodium sulfonate.

4. An aqueous electrolyte for use in lead plating consisting essentially of about 200 grams per liter of lead acetate, about 100 grams per liter of sodium acetate, about 15 grams per liter of acetic acid, about 4 grams per liter of bone glue, about 5 grams per liter of sucrose, and about 5 grams per liter of 2-naphthol-6-sulfonic acid.

5. The process of electrolytically depositing upon a prepared ferrous metal surface a uniform, fine-grained, strongly adhering lead film which comprises the step of contacting said surface as a cathode with an aqueous medium consisting essentially of between about 200 and about 225 grams per liter of lead acetate, between about 60 and about 300 grams per liter of sodium acetate, between about 15 and about 60 grams per liter of acetic acid, between about 4 and about 6 grams per liter of bone glue, and between about 4 and about 10 grams of a watersoluble naphthol sulfonic acid, while said medium contacts'a lead anode and undergoes mild agitation and is impressed with an electric current density of between about and about 140 amperes per square foot of cathode surface.

6. The process of electrolytically depositing upon a ferrous metal surface a fine-grained, tenaciously adhering lead film of uniform thickness which comprises the steps of contacting said surface with 3 to 12% aqueous sulfuric acid at r about 200 F. for between about 30 and seconds, contacting the surface with 8-12% aqueous acetic acid at about 160 F. and then contactin said surface as a cathode with an aqueous solution contacting a lead anode and undergoing mild agitation and consisting essentially of about 200 grams per liter of lead acetate, grams per liter of sodium acetate, 15 grams per liter of acetic acid, 4 grams per liter of bone glue, 5 grams per liter of sucrose, 5 grams per liter of 2-naphthol-8-sodium sulfonate, while an electric current density of about 100 amperes per square foot of cathode surface is maintained in said solution.

7. The process of electrolytically depositing upon a prepared ferrous metal surface a uniform, fine-grained, strongly adhering lead film which comprises the step of contacting said surface as a cathode with an aqueous medium having a pH between about 4.5 and 6.0 and consisting essentially of lead acetate, bone glue and between about 4 and about 10 grams per liter of a water-soluble naphthalene sulfonic acid, while said medium is in contact with lead as an anode and undergoes mild agitation and is impressed with an electric current density of between about 50 and about amperes per square foot of cathode surface.

THERON A. LOVELAND, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,544,726 Colcord July '7, 1925 1,824,100 Schlotter Sept. 22, 1931 2,370,973 Lang Mar. 6, 1945 OTHER REFERENCES Metal Finishing, Oct. 1940, page 533. Transactions, Kansas Academy of Science, vol. 48, No. 2, 1945, pages 169, 172, 1'73. 

